Lecture for Chapter 10
Expression and Regulation
Chapter 10 Outline
• 10.1 How Are Genes and Proteins Related?
• 10.2 How Is Information in a Gene Transcribed
into RNA?
• 10.3 How Is the Base Sequence of a Messenger
RNA Molecule Translated into Protein?
• 10.4 How Do Mutations in DNA Affect the
Function of Genes?
• 10.5 How Are Genes Regulated?
10.1 How Are Genes and Proteins Related?
The Link Between DNA and Protein
•
•
•
•
DNA contains the molecular blueprint of
every cell
Proteins are the “molecular workers” of the
cell
Proteins control cell shape, function,
reproduction, and synthesis of biomolecules
The information in DNA genes must
therefore be linked to the proteins that run
the cell
One Gene Encodes One Protein
• Synthesis of new molecules inside the cell
occurs through biochemical pathways
• Each step in a biochemical pathway is
catalyzed by a protein enzyme
One Gene Encodes One Protein
• George Beadle and Edward Tatum
showed that one DNA gene encodes the
information for one enzyme (protein) in a
biochemical pathway
• There are exceptions to the one gene/one
protein relationship, as discussed later
RNA Intermediaries
• DNA in eukaryotes is kept in the nucleus
• Protein synthesis occurs at ribosomes in the
cytoplasm
• DNA information must be carried by an
intermediary (RNA) from nucleus to
cytoplasm
RNA Intermediaries
• RNA differs
structurally from
DNA
– RNA is single
stranded
– RNA uses the sugar
ribose
– RNA uses the
nitrogenous base
uracil (U) instead of
thymine (T)
RNA Intermediaries
• There are three types of RNA involved in
protein synthesis
– Messenger RNA
– Transfer RNA (tRNA)
– Ribosomal RNA (rRNA)
RNA Intermediaries
• There are three types of RNA involved in
protein synthesis
– Messenger RNA (mRNA) carries DNA gene
information to the ribosome
RNA Intermediaries
– Transfer RNA (tRNA) brings amino acids to
the ribosome
RNA Intermediaries
– Ribosomal RNA (rRNA) is part of the structure
of ribosomes
Transcription and Translation
•
DNA directs protein
synthesis in a two-step
process
1. Information in a DNA gene is
copied into mRNA in the
process of transcription
2. mRNA, together with tRNA,
amino acids, and a
ribosome, synthesize a
protein in the process of
translation
The Genetic Code
• The base sequence in a DNA gene
dictates the sequence and type of amino
acids in translation
• Bases in mRNA are read by the ribosome
in triplets called codons
• Each codon specifies a unique amino acid
in the genetic code
• Each mRNA also has a start (AUG) and a
stop codon (UAA, UGA, UAG)
10.2 How Is Information in a Gene
Transcribed into RNA?
• Transcription of a DNA gene into RNA
has three stages
– Initiation
– Elongation
– Termination
Initiation
1. DNA molecule is unwound and strands are
separated at the beginning of the gene sequence
2. RNA polymerase binds to promoter region at
beginning of a gene on template strand
Elongation
Elongation phase of transcription
1. RNA polymerase synthesizes a sequence of
RNA nucleotides along DNA template strand
2. Bases in newly synthesized RNA strand are
complementary to the DNA template strand
3. RNA strand peels away from DNA template
strand as DNA strands repair and wind up
Elongation
•
As elongation proceeds, one end of the RNA
drifts away from the DNA; RNA polymerase
keeps the other end temporarily attached to
the DNA template strand
Termination
Termination phase of
transcription
– RNA polymerase
reaches a
termination
sequence and
releases
completed RNA
strand
10.3 How Is the Base Sequence of a
Messenger RNA Molecule Translated into
Protein?
mRNA
• An intermediate molecule is required to
convey DNA gene sequence to the
ribosome
• Messenger RNA (mRNA) performs this
function by serving as the complementary
copy of a DNA gene that is read by a
ribosome
mRNA
• In prokaryotes
– The chromosomes are not contained within a
nucleus
– All of the nucleotides in a gene encode for
the amino acids of a protein
mRNA
…In prokaryotes
– Genes for a related
function are
adjacent and are
transcribed together
– Transcription and
translation occur
simultaneously
within the same
compartment
mRNA
• In eukaryotes
– The DNA is in the nucleus and the ribosomes
are in the cytoplasm
– The genes that encode the proteins for a
biochemical pathway are not clustered
together on the same chromosome
mRNA
• In eukaryotes (continued)
– Each gene consists of multiple segments
of DNA that encode for protein, called
exons
– Exons are interrupted by other segments
that are not translated, called introns
mRNA in eukaryotes (continued)
•
•
•
Transcription of a
gene produces a very
long RNA strand that
contains introns and
exons
Enzymes in the
nucleus cut out the
introns and splice
together the exons to
make true mRNA
mRNA exits the
nucleus through a
membrane pore and
associates with a
ribosome
mRNA
•
Why are eukaryotic
genes split up into
exons and introns?
– Through “alternate”
splicing, a cell can
make multiple
proteins from a
single gene*
– *Exception to the
one gene/one
protein relationship
Ribosomes
•
Ribosomes are large complexes of
proteins and rRNA
Ribosomes
•
Ribosomes are composed of two subunits
– Small subunit has binding sites for mRNA and
a tRNA
– Large subunit has binding sites for two tRNA
molecules and catalytic site for peptide bond
formation
Transfer RNAs
•
•
Transfer RNAs hook up to
and bring amino acids to the
ribosome
There is at least one type of
tRNA assigned to carry each
of the twenty different amino
acids
Transfer RNAs
•
•
Each tRNA has three
exposed bases called an
anticodon
The bases of the tRNA
anticodon pair with an
mRNA codon within a
ribosome binding site
Translation
Ribosomes, tRNA, and
mRNA cooperate in
protein synthesis, which
begins with initiation:
1. The mRNA binds to the
small ribosomal subunit
2. The mRNA slides through
the subunit until the first
AUG (start codon) is
exposed in the first tRNA
binding site…
Translation-Initiation
3. The first tRNA carrying
methionine (and
anticodon UAC) binds to
the mRNA start codon
completing the initiation
complex
4. The large ribosomal
subunit joins the
complex
Translation- Elongation
Middle phase of protein
synthesis: Elongation
1. A second tRNA binds to
the second tRNA
binding site adjacent to
the first tRNA
2. The anticodon of the
second tRNA is
complementary to the
mRNA codon exposed
in the second tRNA
binding site…
Translation – …Elongation…
3. A peptide bond
forms between the
methionine and
second amino acid
through the action
of the ribosome
catalytic site
4. The first amino
acid is released
from the tRNA in
the first tRNA
binding site…
Translation…Elongation
5. The “empty” tRNA
in the first binding
site leaves the
ribosome
6. The ribosome
moves down the
mRNA by one
codon, transferring
the tRNA holding
the amino acid
chain to the first
tRNA binding site…
Translation
7. A new tRNA with
anticodon
complementary to the
newly exposed codon
in the second tRNA
binding site
approaches and the
whole elongation
cycle repeats
8. Empty tRNAs are
reloaded with their
appropriate amino
acids by enzymes in
the cytoplasm
End phase of protein synthesis:
Termination
1. A stop codon on
the mRNA slides
into the second
tRNA binding site
2. A special protein
binds to the stop
codon
3. The ribosome
breaks into
separate
subunits…
Translation… Termination
4. The finished
protein chain is
released
5. The mRNA is
released and can
be used to make
another protein
Recap
• Each DNA gene
codes for a single
protein
• Transcription
produces an
mRNA strand
complementary to
the DNA gene
template strand
Recap
• The mRNA strand
associates with a
ribosome
• Transfer RNAs in
the cytoplasm are
loaded with their
appropriate amino
acids by
cytoplasmic
enzymes
Recap
• The ribosome
“selects” the tRNAs
based on the base
pairing of the
anticodon with the
exposed mRNA
codon
• The mRNA contains
start and stop signals
to define where
protein synthesis
begins and ends
Effects of Mutations on Proteins
• Recall that mutations are changes in the
base sequence of DNA
• Most mutations are categorized as
– Substitutions
– Deletions
– Insertions
– Inversions
– Translocations
Effects of Mutations on Proteins
•
Inversions and translocations
– When pieces of DNA are broken apart and
reattached in different orientation or location
– Not problematic if entire gene is moved
– If gene is split in two it will no longer code for a
complete, functional protein
Effects of Mutations on Proteins
•
Insertions or deletions
– Nucleotides are added or subtracted from a
gene
– Reading frame of RNA codons is changed
•
THEDOGSAWTHECAT is changed by deletion of
the letter “S” to THEDOGAWTHECAT
– Resultant protein has very different amino acid
sequence; almost always is non-functional
Effects of Mutations on Proteins
• Nucleotide substitutions (point mutations)
– An incorrect nucleotide takes the place of a
correct one
– Protein structure and function is unchanged
because many amino acids are encoded by
multiple codons
– Protein may have amino acid changes that
are unimportant to function (neutral
mutations)
Effects of Mutations on Proteins
• Effects of nucleotide substitutions
– Protein function is changed by an altered
amino acid sequence (as in gly val in
hemoglobin in sickle cell anemia)
– Protein function is destroyed because DNA
mutation creates a premature stop codon
Mutations Fuel Evolution
• Mutations are heritable changes in the DNA
• Approx. 1 in 105-106 eggs or sperm carry a
mutation
• Most mutations are harmful or neutral
Mutations Fuel Evolution
• Mutations create new gene sequences and
are the ultimate source of genetic variation
• Mutant gene sequences that are beneficial
may spread through a population and
become common
How Are Genes Regulated?
• The human genome contains ~ 30,000
genes
• A given cell “expresses” (transcribes) only a
small number of genes
• Some genes are expressed in all cells
• Other genes are expressed only
– In certain types of cells
– At certain times in an organism’s life
– Under specific environmental conditions
Gene Regulation in Prokaryotes
• Prokaryotic DNA is organized into units
called operons, which contain functionally
related genes
Gene Regulation in Prokaryotes
• Each operon consists of
– A regulatory gene, which controls the
transcription of other genes
– A promoter, which RNA polymerase
recognizes as the place to start transcribing
– An operator, which governs access of RNA
polymerase to the promoter
– The structural genes, which encode for
related proteins
Gene Regulation in Prokaryotes
• Whole operons are regulated as units, so
that functionally related proteins are
synthesized simultaneously when the need
arises
Gene Regulation in Prokaryotes
• The intestinal bacterium Escherichia coli
(E.coli) lives on what its host eats
• Specific enzymes are needed to metabolize
the type of food that comes along
• e.g. in newborn mammals, E.coli are
bathed in milk, containing the milk sugar
lactose
• The lactose operon contains three
structural genes, each coding for an
enzyme that aids in lactose metabolism
Gene Regulation in Eukaryotes
•
Eukaryotic gene regulation
–
–
–
–
DNA is in a membrane-bound nucleus
Variety of cell types in multicelluar eukaryotes
The genome is organized differently
RNA transcripts undergo complex processing
Gene Regulation in Eukaryotes
•
Expression of genetic information by a
eukaryotic cell is a multistep process,
beginning with transcription of DNA, and
ending with a protein that performs a
particular function
Gene Regulation in Eukaryotes
•
Gene expression is
regulated in a number of
ways
–
–
–
–
The frequency of
transcription of a gene can
be controlled
Different mRNAs may be
translated at different rates
Proteins may be
synthesized in an inactive
form and require
modification for activation
Life span of a protein can be
regulated
Gene Regulation in Eukaryotes
•
In eukaryotic cells, transcriptional
regulation occurs on at least three levels
– The individual gene
– Regions of chromosomes
– Entire chromosomes
Gene Regulation in Eukaryotes
•
Regulatory proteins can bind to a gene’s
promoter region and alter transcription
– The protein hormone estrogen causes binding
of a protein to certain gene promoters,
activating transcription
Gene Regulation in Eukaryotes
•
•
Condensed or tightly wound DNA can
make genes inaccessible to RNA
polymerase
Whole chromosomes can be condensed
and inactivated (e.g. Barr bodies in female
mammals)
Gene Regulation in Eukaryotes
• The X chromosome in cats carries fur color
genes
• Female cat cells inactivate one of two X
chromosomes in every cell (producing a
Barr body)
– Different patches of skin cells in a cat inactivate
different X chromosomes
– Patches of fur growing from skin cells may differ
in color if fur genes on X chromosomes differ
Gene Regulation in Eukaryotes
• Patches of different colored fur only occur in
females (e.g. calico cats)